The present invention relates to a method of reducing the surface tackiness of a low-hardness thermally conductive silicone rubber sheet and, more particularly, to a method of reducing evenly the surface tackiness of a low-hardness thermally conductive silicone rubber sheet having strong tackiness so that the sheet surface is uniform throughout.
Since heat-generating electronic parts, such as power transistors and thyristors, suffer deterioration in their characteristics due to the heat generated therefrom, measures to dissipate the heat via, e.g., a heat sink attached to such parts or the metallic chassis installed in the devices utilizing them have so far been taken. Therein, a thermal conductive insulation sheet made of a mixture of silicone rubber with a thermally conductive filler has been arranged between the heat-generating electronic parts and the heat sink.
However, conventional cooling means have become insufficient in many cases because the integration density in electronic devices, such as a personal computer, a word processor and a CD-ROM drive, has greatly advanced in recent years to increase the amount of heat generated from the elements of integrated circuits, such as LSI and MPU, disposed in such devices. Particularly in the case of a note-size portable personal computer, very small inner space thereof makes it impossible to install therein a large heat sink or cooling fan. Further, in such recent electronic devices, the integrated circuit elements are mounted on a printed substrate and the material of the substrate is a glass-reinforced epoxy or polyimide resin having inferior thermal conductivity, so that it is impossible to let the heat escape to the substrate via a thermal conductive insulation sheet, in contrast to conventional devices.
Thus, the method of disposing heat-dissipating parts of spontaneous or forced cooling type in the vicinity of integrated circuit elements to transfer the heat generated from the elements to the heat-dissipating parts is adoptable. According to this method, the heat transfer is not good because of roughness on the interface formed by bringing the elements and the heat-dissipating parts into direct contact. Even when the heat-dissipating parts are attached via a thermal conductive insulation sheet, stresses are set up between the elements and the substrate by the thermal expansion of the thermal conductive sheet because the sheet is more or less inferior in flexibility; as a result, there is a certain danger of the device breaking down.
The attachment of heat-dissipating parts to every circuit element requires excessive space to render it difficult to miniaturize the devices. Therefore, the method of cooling the combination of some elements by the use of one unit of heat-dissipating parts is adopted. In particular, MPU of TCP type used in a note-size portable personal computer is lower in height and greater in the amount of heat generated therefrom than other elements, and so the cooling method applied thereto requires careful consideration.
This creates a demand for thermal conductive materials capable of filling various gaps corresponding to the difference in height among elements. Thus, thermal conductive materials having not only high thermal conductivity but also flexibility enough to fit them into gaps have already been proposed.
For instance, Japanese Tokkai Hei 2-196453 (a term xe2x80x9cTokkaixe2x80x9d as used herein means an xe2x80x9cunexamined published patent applicationxe2x80x9d) discloses a sheet having an easily deformable soft silicone layer laminated on a silicone resin layer prepared by forming a mixture comprising a silicone resin and a thermal conductive substance, such as a metal oxide, into a sheet having strength required for handling. This sheet has almost no tackiness on the side of the silicone resin layer provided with the handling strength, but it has very strong tackiness on the side of the easily deformable soft silicone layer. This strong tackiness becomes unnecessary according to the purposes for or the ways in which the sheet is used.
In addition, Japanese Tokkai Hei 7-266356 and Japanese Tokkai Hei 9-1738 disclose the thermal conductive silicone composite sheets having a thickness of 0.4 mm or below which have a sheet or cloth within for reinforcement and, on at least one side, possesses tackiness and Asker Type C hardness of from 5 to 50. Although the tacky side of these sheets is covered with a protective sheet, the tackiness is unnecessary similarly to the above according to the purposes for or the ways in which the sheets are used. Those attempts to lower the hardness of silicone rubber are, as mentioned above, attended with an undesirable increase in tackiness of the sheet surface.
On the other hand, various methods of reducing the surface tackiness of silicone rubber have hitherto been proposed. For instance, there is known the method in which an organohydrogenpolysiloxane oil is coated on the surface of silicone rubber, and made to undergo a cross-linking reaction with only the vinyl groups present in the vicinity of the surface to lessen the surface tackiness. According to this method, however, the oil coated on the silicone rubber surface by the use of a dipping technique or other conventional technique has a drawback of having difficulty in being coated evenly and render the thickness and the hardness nonuniform.
Further, in Japanese Tokkai Sho 50-87459 was proposed the method of conferring mold releasing properties on a silicone rubber molding (condensation curable type) by dipping the molding in a solution containing an alkoxysilane and a metal salt of organic carboxylic acid or an amine compound in an organic solvent.
Furthermore, in Japanese Tokkai Sho 61-261328 was proposed the method of reducing the tackiness by coating the surface of a cross-linkable organopolysiloxane (in a gelled state) having a cone penetration of from 100 to 350 with an organic peroxide or a photoreaction initiator and then irradiating the coating with ultraviolet rays to cause the cross-linking reaction in the surface part alone.
However, those publications are silent on the manners in which the silicone rubber surface was treated actually, and so it was difficult to reduce evenly the tackiness of a silicone rubber surface so that the surface is uniform throughout.
Therefore, an object of the present invention is to provide a method of reducing evenly the surface tackiness of a low-hardness thermally conductive silicone rubber sheet having strong tackiness so that the sheet surface is uniform throughout.
The aforesaid object of the present invention is attained with a method of reducing the surface tackiness of a low-hardness thermally conductive silicone rubber sheet comprising; coating evenly an organohydrogenpolysiloxane oil on a resin film, pasting the oil-coated side of the resin film to an addition-cured low-hardness thermally conductive silicone rubber sheet having an Asker Type C hardness of from 1 to 80, thereby supplying the organohydrogenpolysiloxane oil to the surface of the silicone rubber sheet, and then causing a cross-linking reaction in the surface part of the sheet.
In accordance with the present method, a resin film and a low-hardness thermally conductive silicone rubber sheet are pasted together after an organohydrogenpolysiloxane oil is coated in a uniform thickness on the resin film, so that the tackiness of the sheet surface can be reduced evenly and the surface treatment for the silicone rubber sheet can be effected by continuous operations to enable mass-production, or a great improvement in productivity.